Electric ventilators of this kind are commonly used in systems for heating or air conditioning rooms or vehicle cabins. In electric ventilators of this kind, the mounting frame is also the part that conveys the air flow produced by the fans.
In prior art electric ventilators, the speed control unit usually comprises a plurality of electrical resistors switched by a remote control selector switch, mounted on the motor's outer casing and suitably insulated from the latter.
This solution is not free of disadvantages: firstly, the resistors are exposed to shocks, dirt, dust, liquids etc.; secondly, the vibrations of the motor casing are transmitted directly to the resistors and, some cases, cause them to be disconnected from the electrical control circuit.
Especially when speed control units of this kind are used in the heating or ventilation systems of earthmoving machines and the like, exposure of the resistors to vibrations, shocks and dirt constitutes a serious drawback.